The etiology of mood disorders is unknown, a critical problem considering their lifetime prevalence of ~20%. Identifying the mechanisms of action of therapeutic agents is one strategy that, combined with other approaches, is important to help clarify the causes of mood disorders and lead to improved therapeutics. Lithium, the classical mood stabilizer used to treat bipolar disorder, inhibits glycogen synthase kinase-3 (GSK3) by a dual mechanism, directly inhibiting enzyme activity and increasing the inhibitory serinephosphorylation of GSK3. Many studies support a role of dysregulated GSK3 in contributing to mood disorders including studies of human postmortem brain, peripheral cells, and genetics, and behavioral studies in GSK3 transgenic mice and with GSK3 inhibitors. This project will extend key findings from our current period of support that provide important links between lithium's inhibition of GSK3 and susceptibility to mood disorders. These topics represents novel findings by this laboratory, indicative of the innovation and productivity of this project, and significantly contributes to understanding the etiology of mood disorders that should help to develop improved interventions. Specific Aim 1 will test if serinephosphorylation of GSK3 regulates mood-relevant behaviors. We will use isoform-selective GSK3&#945;21A/21A/&#946;9A/9A knockin mice, where the regulatory serine of either GSK3 isoform is mutated to alanines, for behavioral tests to clarify the regulation of mood relevant behaviors by active GSK3. Since neurogenesis may to be involved in mood disorder susceptibility and/or therapeutic responses, Specific Aim 2 will test if GSK3 regulates the plasticity of neurogenesis. We will test if GSK3 knockin mice display altered neurogenesis increased by exercise and impaired by chronic stress, relate to mood-relevant behaviors, and examine mechanisms. Vulnerability to mood disorders is greatest during development. Since we found GSK3 levels are elevated several-fold in juvenile and adolescent mouse brain, the increased GSK3 may contribute to developmentally-regulated susceptibility to mood disorders. Specific Aim 3 will test mechanisms by which dysregulated GSK3 may be involved in mood-relevant behaviors and responses to therapeutic drugs. We will test if the rapid antidepressant effect of ketamine involves inhibition of GSK3, test if neurotrophins or inflammatory molecules are altered by GSK3 and associated with susceptibilities to mood-relevant behaviors and neurogenesis studied in SA1 and SA2, and examine changes in GSK3 association in protein complexes that regulate signaling. Specific Aim 4 will test the developmental profile of heightened susceptibility to depression-like behavior and hyperactivity in GSK3 knockin mice and developmental responses to drugs. Altogether, this project will continue to address important problems concerning the causes and treatments of mood disorders.